The present invention relates to a system for homogenizing a laser pulse emitted by a laser source in order to illuminate a target homogeneously.
It finds a particularly useful application in the field of intense laser pumping of femtosecond-amplifier solid media.
In fact, the homogeneity of laser pumping of femtosecond amplifiers that are based in particular on Ti:Sapphire technology is an essential parameter, owing to the difficulty of pumping the amplifiers efficiently (from the standpoint of extraction of energy), and robustly (from the standpoint of damage to the laser material). The damage threshold of Ti:Sapphire, in nanosecond mode, is estimated between 5 and 10 J/cm2. This value does not follow from a systematic study but from an estimate based on experience. The uncertainty concerning the value of the threshold obliges laser engineers to pump the active materials just beyond the saturation fluence, i.e. 1 J/cm2, with a low efficiency of energy extraction (20%), in order to protect the crystals from the risk of damage.
In the case of pumping at 4 J/cm2, for example, the efficiency would be three times higher (60%). In such a case, the damage threshold of the material would be very close to the pumping fluence required for good extraction of energy in power amplifiers, and any inhomogeneity of the pump laser would be translated into a risk of major damage, therefore to a considerable reduction of reliability or consequently to a degradation of performance, both of which mean appreciable extra costs.
This inhomogeneity is due to pump profiles that are never completely under control with the imaging techniques currently in use. These profiles have modulations or hot spots that are often significant and therefore often lead to the destruction of the laser crystals, which moreover are excessively expensive, their price reaching several tens of thousands of Euros for the largest ones, and take a long time to grow, up to 1 year.
If we bear in mind that from 60% to 70% of the price of a laser amplifier is due to the pump lasers, it is clear that this identifies, in the control of the spatial quality of pumping laser beams, a major parameter for the development of systems with better performance.
For limiting the modulations in the pump profiles, we might consider the fact that when the beam is propagated over a few metres, without leaving the Rayleigh zone, the modulations are minimal. For example, if we consider a pump laser supplying 1 J/cm2 (at 532 nm and at 10 Hz) to a Gaussian beam with a diameter of 10 mm, the Rayleigh zone will extend to 140 m. In reality, however, the typical profiles of pump lasers are closer to a “top-hat” (super-Gaussian) profile than a Gaussian profile, as this form is required for optimizing the frequency conversion and the performance of the source in terms of useful energy. In the case of a super-Gaussian profile, with the other characteristics of the example considered above, modulations already appear after just four metres of propagation and can become dangerous for the optics and the laser crystals, even if the Rayleigh zone is much longer. Owing to these modulations it is difficult in practice to use a propagation distance greater than 3-4 metres. With the propagation approaching the Rayleigh zone, there are therefore appreciable constraints on the configuration of the amplifiers, because of the small distances over which beam homogeneity is effectively conserved.
Otherwise, for limiting the modulations in the pump profiles, it is also possible to use the technique consisting of transporting the near field (assumed homogeneous) of the pumping laser beam, by imaging on the amplifying crystal. But this technique does not in any way protect the latter from any possible variation in intensity that might develop over time on the pumping laser used.
It is now known that for effectively limiting these modulations in the pump profiles, it is necessary to homogenize the source, which implies complete control of the transport of energy, from the pump lasers to the active material. With such control, it would be possible to pump the lasers in conditions with high extraction efficiency, but close to the damage threshold and avoid the hot spots that can damage the amplifying crystals. Generally, the purpose of a homogenizer is to ensure homogeneous energy distribution on the amplifying crystal regardless of the initial spatial distributions of the incident beams.
In this connection, a refractive homogenizer with matrices of microlenses is known. Said refractive homogenizer with subpupils is constituted by two parts: a matrix optical element composed of a set of microlenses and a focusing component. Each microlens represents a subpupil. The microlenses separate the incident beam into several segments, and the focusing component superimposes the projection of each subpupil on the focal plane. This technique is based on the low spatial coherence of the laser beam at the entrance to obtain an averaging effect (intensity sum) of the contributions of the various sub-elements distributed over the entire zone to be pumped.
This technique has shown good performance in pumping systems with low spatial coherence. However, for pumping systems with almost perfect spatial coherence, the matrix of microlenses induced modulations of 100%. Even in the case of beams with low spatial coherence, the best homogenization performance is obtained outside of the focal plane, as the latter is modulated by the diffraction effects of the periodical structures, called the Talbot effect. In other words, this Talbot effect prevents the use of the energy distribution in the focal plane and makes it necessary to use planes where the energy distribution is slightly modulated but is not so close to the ideal “top-hat”.
Document U.S. Pat. No. 4,521,075 is known; this describes a system for rendering a laser beam, directed towards a target, spatially incoherent. This system comprises, between the laser source and the target:                an optical component for introducing spatial incoherence between several parts of the laser beam, and        a focusing lens for directing the laser beam on the target. Overlapping then occurs, which limits the interference at the target.        
However, experience has shown that this system is unable to homogenize a source other than a very incoherent laser, for example laser diodes and excimer lasers.